Richard Hicks

Direct comparison of corticospinal volleys in human subjects to transcranial magnetic and electrical stimulation

1. The effects of graded transcranial magnetic and anodal electrical stimulation of the human motor cortex were compared in human subjects undergoing orthopaedic operations on the spine, before and after withdrawal of volatile anaesthesia. Corticospinal volleys were recorded from the spinal cord in the low‐cervical and low‐thoracic regions (six subjects) or the mid‐thoracic region (two subjects) using bipolar electrodes inserted into the epidural space. 2. Electrical stimuli were delivered using anode at the vertex and cathode 7 cm laterally. The corticospinal volley at threshold consisted of a single deflection with a mean latency to peak of 4.17 ms at the rostral recording site. With further increases in stimulus strength the latency of this D wave shortened in two steps, first by 0.89 ms (seven subjects) and then by a further 0.8 ms (two subjects), indicating that the site of activation of some corticospinal neurones had shifted to deep subcortical sites. 3. When volatile anaesthetics were given, a corticospinal volley could not be defined in three subjects with magnetic stimuli of 70, 80 and 100% maximal stimulator output with the coil at the vertex (Novametrix Magstim 200, round coil, external diameter 14 cm). In the remaining five subjects, the component of lowest threshold was a D wave recorded at the rostral site at 4.0 ms when stimulus intensity was, on average, 70%. With stimuli of 90‐100% a total of five small I waves could be defined in the five subjects (i.e. on average one I wave per subject). 4. After cessation of volatile anaesthetics in seven subjects, the thresholds for D and I waves were lower and their amplitudes were greater. The D wave remained the component of lowest threshold in all subjects, appearing at the low‐cervical level with magnetic stimuli of 50%. However, in three subjects I waves also appeared at D wave threshold, and the D wave was smaller than with electrical stimulation at I wave threshold. There was no consistent change in latency of the magnetic D wave as stimulus intensity was increased to 100%. 5. These findings suggest that the previously reported difference in latency of the EMG potentials produced in upper‐limb muscles by anodal stimulation and magnetic stimulation of the human motor cortex is not because the corticospinal volley induced by magnetic stimulation lacks a D wave.(ABSTRACT TRUNCATED AT 400 WORDS)

Transcranial electrical stimulation of the motor cortex in man: further evidence for the site of activation.

1. The motor cortex was stimulated electrically (vertex anode; cathode 6 cm lateral) in neurologically normal subjects undergoing surgery for scoliosis, and the evoked corticospinal volleys were recorded from the spinal cord using epidural electrodes. 2. Stimuli > 330 V produced a complex D‐wave volley containing three separate peaks, with high‐threshold components, 0.8 ms (D2) and 1.6 ms (D3), in advance of the lowest‐threshold component (D1). As stimuli increased up to 1500 V, D3 replaced the later components completely, but there was no further latency ‘jump’. 3. Brainstem stimulation using electrodes over each mastoid process produced a descending volley that had the same latencies as D3. At threshold, stimulation of the brainstem or spinal cord attenuated the D wave evoked by simultaneous cortical stimulation. 4. It is concluded that transcranial electrical stimulation of the motor cortex at high intensities can access corticospinal neurones at the pyramidal decussation, and that stimulation of the brainstem (and the spinal cord) preferentially accesses corticospinal axons. At threshold, motor cortex stimulation probably activates corticospinal neurones at or near the cerebral cortex.

Corticospinal volleys evoked by anodal and cathodal stimulation of the human motor cortex.

1. In fifteen neurologically normal subjects, corticospinal volleys evoked by transcranial stimulation of the motor cortex were recorded from the spinal cord using epidural electrodes in the high‐thoracic and low‐thoracic regions during surgery to correct scoliosis. 2. Anodal stimulation at the vertex produced complex corticospinal volleys that could be recorded at both sites, with multiple waves analogous to the D and I waves documented in animal experiments. These volleys were of higher amplitude when the cathode was 7 cm lateral to the vertex rather than 7 cm anterior. There were no differences in conduction time between the two recording sites for D and I waves, when these waves could be identified at the low‐thoracic site. 3. Anodal stimuli of 150 V commonly produced a descending volley containing a single peak at both recording sites. Modest increases in stimulus intensity to 225‐375 V produced a peak 0.8 ms in advance of the wave of lowest threshold in thirteen subjects and, in seven subjects, further increases produced an additional peak 1.7 ms in advance of the first‐recruited wave. The early peaks increased in size with stimulus intensity, replacing the first‐recruited wave. These results suggest that the site of impulse initiation with electrical stimulation of the motor cortex shifts from superficial cortex to deep structures, approximately 5 and 10‐11 cm below the cortex. These sites are probably the internal capsule and the cerebral peduncle. 4. With cathode at the vertex and anode over the ‘hand area’ the response of lowest threshold occurred at the latency of the anodal D wave but could not be recorded at the low‐thoracic site, suggesting that it was generated by the anode over the ‘hand area’. Slightly higher intensities induced a ‘cathodal D wave’ and still higher intensities produced late peaks at latencies of anodal I waves. These cathodal D and I waves involved axons innervating lumbar segments. There was no evidence that cathodal stimulation preferentially produced I waves. Cathodal stimulation at the vertex with the anode 7 cm anteriorly produced similar results: D waves were produced at relatively low intensities, but I waves appeared at relatively high stimulus intensities if at all.(ABSTRACT TRUNCATED AT 400 WORDS)

Variability of Motor-Evoked Potentials Recorded During Nitrous Oxide Anesthesia from the Tibialis Anterior Muscle After Transcranial Electrical Stimulation

When recorded as a compound muscle action potential (CMAP), the motor-evoked potential (MEP) is affected by volatile anesthetics and nitrous oxide. However, MEPs recorded using epidural electrodes in the presence of nitrous oxide are highly reproducible from trial to trial. We wished to establish the reproducibility over time of the CMAP produced by supramaximal transcranial electrical stimulation of the human motor cortex. Cascades of 100 successive CMAPs were recorded from the tibialis anterior muscles of six anesthetized patients undergoing scoliosis surgery, in response to transcranial electrical stimuli of >500 V. Satisfactory CMAPs could be recorded in the presence of nitrous oxide, but not isoflurane. Latencies and amplitudes were reproducible in repeated sequences of 100 responses. However, amplitude and, to a lesser extent, latency, were highly variable within a sequence. In addition, occasional individual stimuli, although rarely successive ones, failed to evoke a CMAP. CMAPs have a much higher trial-to-trial variability than corticospinal volleys recorded from the epidural space. Using the present methodology it would be difficult to rely on CMAP recordings as an indicator of corticospinal function in the clinical monitoring situation. (Anesth Analg 1996;82:744-9)

Corticospinal volleys evoked by electrical stimulation of human motor cortex after withdrawal of volatile anaesthetics.

1. In twenty‐two neurologically normal patients undergoing surgery for scoliosis, corticospinal volleys to transcranial electrical stimulation of the motor cortex were recorded from the spinal cord using epidural electrodes. While anaesthesia was maintained by nitrous oxide and narcotics, volatile anaesthetics were withdrawn to determine whether such agents had a depressant effect on the evoked corticospinal volley. 2. Profound changes were documented in liminal D waves, there being an increase in amplitude averaging 392% following withdrawal of the volatile anaesthetic. There was a proportionately smaller increase (averaging 26%) in supraliminal D waves; these had a complex bifid or trifid shape indicating that some corticofugal axons were being activated deep to cortex. In general the effect on the D wave of withdrawing the anaesthetic agent was similar to that of increasing stimulus intensity. 3. Withdrawal of isoflurane dramatically increased the number of I waves and their mean amplitude. In the absence of isoflurane, I3 (mean latency 3.5 ms after the D wave) became the dominant I wave. The amplitude of I2 (mean latency 2.2 ms) became slightly smaller. The change in I waves could not be likened to an increase in stimulus intensity, because I waves invariably increase in, or remain of the same, amplitude as stimulus intensity is increased. 4. These findings indicate that changes in motor cortex excitability can result in major changes in the corticospinal volley produced by transcranial electrical stimulation, affecting both the D wave and I waves. They caution against identifying a cortical action solely on the basis of a change in the responses to magnetic stimulation of motor cortex but no such change to electrical stimulation.

Trial-to-trial variability of corticospinal volleys in human subjects

The trial-to-trial variability of the different components of corticospinal volleys evoked by transcranial electrical stimulation of the motor cortex using a constant stimulus intensity was measured from epidural recordings during surgery to correct scoliosis. The recordings were made when there was no operative interference, and blood pressure, temperature, ventilation and anaesthetic regimen were stable. A simple D wave with a single negative peak of 10-30 microV amplitude was recorded in 4 patients. It varied little in amplitude (S.D.s < 8% for 100 consecutive single responses). In 4 patients the stimulus was adjusted to produce a complex D wave with 3 components, the earliest 2 of which arise from subcortical/brain-stem sites. The variability of amplitude of these components was high (S.D.s of 13-50%), but the variability of latency was low (S.D.s of 2-3%). Eighteen I waves were recorded in 6 of the subjects. Their variability from trial to trial was similar to that of the components of the complex D wave. It is argued that there would be greater trial-to-trial variability of the corticospinal volley in the awake state, particularly when the stimulus was magnetic rather than electrical. Explanations for changes in the compound muscle action potential produced by transcranial stimulation, electrical or magnetic, must take into account that a constant stimulus does not evoke an identical descending volley.

Cotrel-dubousset instrumentation in children using simultaneous motor and somatosensory evoked potential monitoring.

Study Design To record prospectively combined motor- and somatosensory-evoked potentials in children during scoliosis surgery using Cotrel-Dubousset instrumentation, without using special anesthetic or muscle relaxant regimens. Objective To determine the outcome of scoliosis surgery guided by a new technique of monitoring motor- and somatosensory-evoked potentials simultaneously. Summary of Background Data Other techniques used to assess cord function generally are limited by special anesthetic requirements or assess only a limited part of the cord or monitor motor function separately from somatosensory function. Methods Spinal cord function was monitored using epidural leads to record simultaneously the descending motor volley (by transcranial electrical stimulation) and the ascending somatosensory volley (by tibial nerve stimulation) at two spinal levels. Results Combined motor- and sensory-evoked potentials were recorded successfully in 138 of 160 children (81%). Changes in evoked potential waveforms were seen in eight patients (5%), but resolved or lessened in response to appropriate measures. Curve correction was satisfactory, and there were no new postoperative deficits or worsening of preexisting deficits in any patient. Conclusion A spinal cord monitoring system is described that is safe, reliable, accurate, and makes it unnecessary to resort to the “wake-up” test.

Corticospinal volleys evoked by transcranial electrical and magnetic stimulation

Monitoring corticospinal function during surgery is now feasible and a number of different techniques have been implemented by different authorities, as discussed below. Based on recordings of somatosensory volleys using recording electrodes inserted into the epidural space, as pioneered by Jones and colleagues (1982, 1983), and the first report of corticospinal volleys recorded using similar electrodes (Boyd et al., 1986), our unit has developed a recording system that allows the simultaneous recording of descending corticospinal volleys and ascending somatosensory volleys from the spinal cord (Fig. 1), and has used it extensively, primarily during scoliosis surgery (Hicks et al., 1991; Burke et al., 1992b; Stephen et al., 1996). This chapter addresses the nature of the corticospinal volleys set up by electrical and magnetic stimulation of the human motor cortex through the scalp, as recorded using this technique. The data are relevant not only for those who monitor spinal cord function during surgery but also for those who use transcranial stimulation of the motor cortex as a diagnostic or research tool.